Add markdown toolbar

[onukura]

Before submitting a pull-request to GitBucket I have first:

• read the contribution guidelines
• rebased my branch over master
• verified that project is compiling
• verified that tests are passing
• squashed my commits as appropriate (keep several commits if it is relevant to understand the PR)
• marked as closed using commit message all issue ID that this PR should correct

About this PR

revised #2517 . This PR add markdown toolbar for editor to resolve #1793 and might help #1411.

@takezoe This PR also fixed wired behaviors you mentioned in the last comment except the one below.

Undo doesn't work after modifying text using toolbar

[takezoe]

This works very well!!

One thing I noticed is that it would be better the insert code button can quote multi-lines code fragment properly. In GitHub, it quotes with a single quote when a single line code fragment is selected and quotes with triple quotes when multi-lines are selected. But even without it, this is much beneficial to edit Markdown content.

@onukura Do you afford to update this to support quoting multi-lines code fragment? Otherwise, I will merge this as it is.

[onukura]

Do you afford to update this to support quoting multi-lines code fragment? Otherwise, I will merge this as it is.

@takezoe I didn't notice this... I think it's better to include in this PR.
And another missing feature is "undo". I am concerned that missing "undo" will cause poor usability...

Anyway "multi-line code fragment" and "undo" will take time to implement. I will try to find time to handle them.
However if next release 4.35.0 is around the corner, this feature might be in 4.36.0.

[takezoe]

However if next release 4.35.0 is around the corner, this feature might be in 4.36.0.

Got you. I will exclude this from 4.35.0 if this pull request is not ready to merge by the next release.

[takezoe]

Moved to 4.36.0.